With the coming of an era of high oil prices, seeking a variety of alternative energy sources has become imperative. At present, the carbon volume of gas hydrates in global seabed is two times as much as that of carbon in coal, oil and natural gas. Thus, the gas hydrate has become an important reserve energy which must be considered in the balanced development of energy strategic. The total volume of gas hydrates in South China Sea is probably half as much as that of oil and natural gas in onshore and offshore of China. Therefore, as an alternative source of energy, the development and utilization of gas hydrates have been increasingly emphasized by people in the world. Gas hydrates are mainly distributed in the seabed sediment of the ocean edge and permafrost horizon of onshore. Sample drilling is a most direct way to identifying gas hydrate, and is also the essential method to verifying the investigation achievement of other ways. So, sample drilling has great significance for the development of gas hydrates.
Gas hydrates are formed under the condition of low temperature and high pressure, such special condition needs high requirements on sample drilling. At present, the pressure-tight core barrel is a main truth-preserving core sampling tool, and its design mentality is that when the hydrate core (or core sample) has entered into the core chamber of the pressure-tight core barrel, the bottom of the core barrel is closed by a ball valve to enable the core to maintain its initial pressure, and a pressure compensation device is used to control the pressure in order to maintain it unchanged during the whole process of lifting the core from the hole bottom, and then a freezing preservation process is carried out after the core has been lifted to the ground. The heat preservation method of the core in the drill hole mainly uses thermal insulation materials to achieve passive thermal insulation. This method is used to inhibit hydrate decomposition by mechanically maintaining a constant pressure, the requirements for the strength of whole core, especially strength and sealing of the ball valve are quite high. Even if the sealing performance of the ball valve decreased slightly, the core would not be able to maintain the initial pressure, which may cause a failure of coring. When the design pressure of the sampler reaches a certain level, the materials and sealing performance of the sampler need to be improved a lot if the pressure is desired to be further increased, but it is not easy to be achieved.